The invention relates to processing information received over a communication channel, for example, from a data storage system. In particular, the invention relates to systems, methods and apparatuses for the accurate retrieval of data from optical media by controlling the amplitude of a read signal applied to read signal processing apparatus.
Storage technologies have evolved to meet the ever-increasing high capacity data storage needs of consumers and businesses. The creation, storage, use, and retrieval of digital entertainment content and business data are but a few examples of such needs. Although magnetic, optical and magneto-optical technologies have to some extent addressed the current needs related to storage capacity, technologies such as high-definition video generally demand ongoing improvements to the retrieval and processing functions to keep pace with the speed with which the data must be processed to take full advantage of such technologies. Furthermore, imperfections and normal variations in the media as manufactured, as recorded or embossed, or resulting from handling, may introduce unwanted errors. The accurate retrieval and processing of data, in the face of errors caused by scratches, poor recording, micro-scratches, inexpensive disks, high recording densities and other performance characteristics continues to be of concern.
Optical disks such as CD's, DVD's, and newer digital formats such as HD-DVD and Blu-Ray provide numerous benefits over traditional magnetic media such as VHS tapes and diskettes due to their durability and the high data capacities they provide. The significant increase in processing capabilities of even the most inexpensive personal computers, as well as the almost universal acceptance of DVD as the media format of choice has provided consumers with the ability to enjoy movies and other forms of audio-visual entertainment in their homes at both an extremely high quality and at a reasonable cost.
To retrieve information, a conventional optical storage and retrieval device utilizes a solid state laser to illuminate the storage media and detect variations in one or more physical properties of the storage media at a desired location with respect to the disk's surface. For example, for DVD media, marks and spaces are recorded or embossed 0.6 mm below the storage media protective substrate surface as regions of different reflectivity or different depths. New, proposed media types place the marks 0.1 mm below the surface. When data is stored in the form of mark depth, phase interference techniques are employed to convert varying depth into detectable phase variations in the reflected light. The contrast ratio between mark regions and space regions, whether phase differences or reflectivity differences, needs to be sufficiently high to be detectable. The contrast ratio observed is highly dependent on writing conditions and reading conditions, as well as the characteristics of the disk as manufactured. For example, interference techniques rely on phase differences of about 180°, which are the result of mark depths, in such media, of about ¼λ, where “λ” represents the wavelength of the light used to read the mark. For red laser light having a wavelength of about 640 nm, ¼λ is about 180 nm. During reading, the read laser is focused on the marks, so as to detect the 180 nm height difference, while most DVD media surface defects are 0.6 mm away from the marks being detected, and so are well out of focus. The operation and performance of the storage and retrieval device is thus highly dependent upon the properties of the storage media, the condition of the storage media (e.g., dirty, scratched, manufacturing quality, etc.), the properties of the apparatus in which the storage media was recorded or embossed, the precision of the apparatus in which the storage media was recorded or embossed and the properties and precision of the stored data retrieval device.
In optical data storage systems, several factors are coordinated in order to accurately retrieve data from the media, including in some systems, but not limited to, the speed of the disk, delays in electronics, operational characteristics of the laser, the speed of related systems, etc. Consequently, timing clocks are often coordinated or locked together to provide overall control of data timing in storage systems. In addition to timing, gains and offsets of the various electronics are controlled and coordinated so that information read from a disk accurately represents the data recorded or embossed on the disk. Ineffective control and settings can impede the operation of these electronics, and result in retrieved data inaccuracies and the inability to operate properly.
In order to more clearly disclose aspects of the invention, certain terms are now defined, and used as defined throughout this disclosure and claims.
Definition List 1TermDefinitionSaturation levelWhen applied to an input signal that mayassume a variable value, this refers to avalue defining a boundary, to one side ofwhich a component acting on the signalacts according to its designed transferfunction, and to the other side of whichthe component enters a limitingoperating region. Componentsfrequently exhibit both an upper and alower saturation level, operatingaccording to the designed transferfunction between them. In the case of astepwise linear component, such as ananalog-to-digital converter, thecomponent has a stepwise linear transferfunction between two saturation levelsand the transfer function becomes non-linear beyond either saturation level.When applied to an output signal, thisrefers to the value assumed by theoutput signal when the componentproducing the output signal is operatingin a limiting operating region.Peak-to-peakThat signal amplitude having peaksaturation levelexcursions just touching both upper andlower saturation levels.Non-predeterminedContent of a signal that is notinformation contentdetermined beforehand. An example ofnon-predetermined information contentis user data. Some examples ofpredetermined information contentinclude the data defining a sync patternand header address information.Channel bit (T)A channel bit (T) in a signal, defined overa span of either space or time, is thespacing in either space or time thatdefines where transitions defining edgesof each symbol represented in the signalcan legitimately occur. In some codes,transitions may not fall less than threechannel bits apart, but may be three,four, five, . . . channel bits apart, thus theclosest difference between symbollengths is one channel bit (1T).Medium (media)The physical, information-carryingobject(s) through which, or on which,information is communicated or stored.In communication systems, mediainclude, but are not limited to, copperwire and fiber optic materials. In storagesystems, media include, but are notlimited to, CD disks, DVD disks and thelike.
Conventional optical disk storage read channels, 101, employ Automatic Gain Control (AGC) systems 100, such as the one depicted in FIG. 1. AGC system, 100, employs a circuit topology known as a feedback connection or closed loop circuit to continuously monitor a signal 10 and to adjust the gain of one or more elements in response to the signal. For example, the AGC system, 100, may include a Programmable Gain Amplifier (PGA), 102, and the AGC system, 100, may be a closed loop circuit including an AGC circuit, 104, that detects the amplitude of an input signal 10 to the read channel, 101, and adjusts the PGA gain to maintain a chosen amplitude of signal 10. Most conventional AGC circuits, 104, measure maximum and minimum signal levels, and ensure that a signal whose amplitude is below the peak-to-peak input saturation level of the downstream components is delivered to those downstream components, such as an analog-to-digital converter (ADC), 103, thus avoiding driving the output signal, 10, of the ADC, 103, into or beyond the output saturation level of the ADC 103.
The AGC system, 100, may have a programmable response time. Time constants of conventional AGC systems, 100, may be on the order of hundreds of channel bits, T, or even longer, making conventional AGC systems, 100, intentionally slow to react, so as to improve the stability of the signal subjected to AGC control.
In conventional optical storage systems employing run-length limited (RLL) modulation, the stored information resides in the pulse width of the RLL modulated signal, rather than in its amplitude. For conventional DVD and CD formats, the RLL modulation pulse widths, as recorded or embossed on the disk and seen in the readout signal, vary between a minimum of three channel bits (3 T) and a maximum of eleven channel bits (11 T) in length. The AGC adjusts gain so as to prevent the ADC's digital output from reaching the saturation level, even on long (e.g., 11 T) symbols, causing extraction accuracy of the data carried by short (e.g., 3 T) symbols to decline as a result of the limited number of ADC quantization levels available to represent the shorter symbols, which have lower amplitudes. Because the time constant of the AGC may be hundreds of channel bits, T, or longer, the AGC is not designed to, and does not react to the rapid changes that occur when a 3 T, or other short symbol is followed by an 11 T, or other long symbol or vice versa.
The effect of a conventional AGC system on a readout signal is shown in FIG. 3. A readout signal, 301, is determined by the conventional AGC to exceed upper and lower thresholds, 302 and 303. The upper and lower thresholds, 302 and 303, represent the input signal saturation levels for a component (e.g., FIG. 1, ADC, 103) receiving the readout signal, 301. The AGC lowers the gain of a Programmable Gain Amplifier (PGA) through which the readout signal, 301, passes, producing the output signal, 304. The output signal, 304, does not exceed the saturation levels, 302 and 303, anywhere. Ideally, in the conventional AGC, the output signal, 304, just reaches the saturation levels, 302 and 303, but does not exceed the saturation levels, 302 and 303. In practical systems, the peak-to-peak amplitude of the output signal, 304, would be set to some marginally lower amplitude, say 95% of peak-to-peak saturation level, to allow for some variations in the maximum level of the input signal. Because longer pattern symbols are lower frequency waveforms than shorter pattern symbols, and because lower frequencies are less attenuated during transmission as a result of intersymbol interference, longer pattern symbols have higher amplitudes than shorter pattern symbols. The sampled values, 305, which are both sampled in time and quantized to discrete quantization levels by an ADC (e.g., FIG. 1, 103), adequately represent longer pattern symbols, 306, which have high amplitudes, but do not have sufficient quantization resolution to adequately represent shorter pattern symbols, 307, which have substantially lower amplitudes.
As seen in the illustrative example of FIG. 3, for short symbols, 307, samples, 308, in the peak amplitude region are quantized to one level, while in the region of the symbol, 307, near the zero-crossing points, samples, 309, are quantized to another level, in this example, to zero. Because plural samples, 309, in the vicinity of the zero-crossing points are all quantized to one level, in this example zero, the precise location of the zero-crossing point is obscured. In practical systems, samples near the zero-crossing point may be quantized to levels other than zero, but nevertheless close enough to zero that they tend to make accurate interpolation of the time of the zero-crossing point difficult, if not impossible.
Further, conventional AGC approaches cannot respond to defects such as manufacturing surface defects, scratches, fingerprints, smudging, etc., that are shorter than or up to the order of magnitude of the AGC's response time. Furthermore, prior long and high amplitude pattern symbols may result in reduced amplitude resolution of short pattern symbols at the ADC output during normal operation. This effect of poor performance when long patterns are followed by short patterns is exacerbated in the presence of defects. This additionally reduces the read channel's ability to accurately extract data.
Known AGC systems are disclosed in U.S. Patent Application No. 2003-0079161A1, filed by Verboom, U.S. Pat. No. 6,621,338, issued to Van Schyndel, and U.S. Pat. No. 6,091,687, issued to Verboom et al. U.S. Patent Application No. 2003-0079161A1 discloses a conventional AGC that monitors a special signal carrying predetermined information content specially written in the user data area for the AGC to act upon. U.S. Pat. No. 6,091,687 discloses use of an AGC to control signal levels in communication systems.